1. Field of the Invention
The present invention relates generally to printing presses and more particularly to a device and method for a providing ink or other solutions to a plate cylinder or image member of a printing press.
2. Background Information
Offset lithographic printing presses, for example, typically have a plate cylinder carrying an imaged plate. During printing, the plate is inked, and the inked image is transferred to a blanket which then contacts paper sheets or a continuous web of paper. Ink for the plate cylinder may be transferred from a fountain roll to an inker roll by a series of metering rolls and belts. From the inker roll the ink is transferred to the plate cylinder.
One way to meter ink from the fountain roll to the inker rolls is via ductor and slipping-nip metering rolls. However, the metering by ductor and slipping-nip metering rolls may result in a discontinuous ink transfer. Also, both ductor and metering rolls have gross slip between the roll surfaces during the transfer process.
Czechoslovakian Patent Document No. 248128 purports to disclose an inking apparatus with a flexible and elastic belt. Ink is applied to the flexible and elastic belt as a uniform and broad strip. Two tensioning rollers are connected to an automatic tension control, which the patent asserts move apart from each other to form a thinner ink layer or toward each other to form a thicker ink layer. A compacting roller contacts one of the tensioning rollers.
The device of the 248128 Czech Patent Document has the disadvantage that the tensioning rollers, compacting roller and device for automatic tension control require that a path of the belt be expanded or decreased to alter the ink thickness, as the automatic tension control varies a distance between the two tensioning rollers. The changing of the path length for a flexible elastic belt requires a complicated arrangement and might lead to increased belt failure. Moreover, the tensioning system is a complex arrangement requiring tensioning rollers and compacting roll to provide a sufficient tension in the elastic belt. In addition, it does not appear as though the surface speeds of the rollers of the Czech patent device vary.
An object of the present invention is to provide for an accurate and reliable device and method for providing continuous ink or other solution metering for a printing press.
The present invention provides a device for providing ink or another solution to a printing press comprising at least one metering device, each metering device having:
a first roll having a first surface speed;
a second roll having a second surface speed lower than the first surface speed; and
an elastic belt running in a fixed-length path over the first roll and the second roll.
The first roll has a higher surface speed than the second roll, and thus defines a high speed roll, while the second roll defines a low speed roll. The elastic belts are arranged such that each elastic belt rotates preferably around only one low speed roll and one high speed roll.
xe2x80x9cFixed-length pathxe2x80x9d as defined herein means that the belt path length does not essentially change as the belt traverses its path, although minor variations due to the stretching and contracting of the belt could occur.
With the elastic belt of the present invention, discontinuous ink transfer can be avoided. Also, the elastic belt avoids the gross slip between the roll surfaces that occurs sometime during the transfer process. Thus, the present invention provides the advantage of continuous non-slip ink metering. Moreover, the elasticity of the belt allows the differing surface speeds of the rolls to deform the elastic belt.
Preferably, the belt experiences no slip at either the first or second roll. The no-slip contact results in more consistent printing. xe2x80x9cNo slipxe2x80x9d as defined herein means that although a micro-creep may occur, the belt does not slip to as to significantly alter the speed of the belt with respect to the surface speed of the roll.
Each metering device may be arranged so that it may contact another elastic belt at a nip.
Preferably, the elastic belts have an ink film on the outer surface. The thickness of the ink film has an inverse relationship to the amount of strain in the elastic belt.
The elastic belts elongate and become thinner on exiting the roll with the lower surface speed, and compress and thicken on exiting the roll with the higher surface speed. The change in the elastic belt provides the advantage of allowing the elastic belts to deform in response to stress produced by the differing speeds of the rolls. Furthermore, the deformation of the elastic belts changes the surface area of the elastic belt, causing the thickness of the ink film to change and affecting the distribution of the ink on subsequent devices. Because the rolls have different surface speeds, the surface speed of the elastic belts varies.
The radius of the second roll is preferably, but not necessarily, smaller than the radius of first roll. The differing radii provide the advantage of changing the surface speeds of the rolls, which in turn affects the deformation of the elastic belt. The rolls may then have the same rotational speed, which can provide for simpler gearing mechanisms, e.g., 1-to-1. Equal sized rollers could also be used if driven at different rotational speeds.
The present invention may include a fountain roll that contacts one of the low speed rolls, so as to form a nip with one of the first and second rolls. The fountain roll provides the advantage of allowing the ink film to be transferred to the elastic belt.
The present invention may also comprise an inker roll that contacts one of the rolls, so as to form a nip. Also, the present invention may include a print cylinder, such as a plate cylinder, that may contact the inker roll, so as to form a nip.
The present invention also includes a method for inking or providing another solution to a cylinder comprising the steps of:
rotating a first roll at a first surface speed;
rotating a second roll at a second surface speed, the second surface speed being less than the first surface speed;
moving an elastic belt over the first and second rolls, the elastic belt maintaining a fixed path length; and
contacting the belt with ink.
The first and second rolls preferably are driven rolls.
The present invention also provides a device for providing ink or another solution in a printing press comprising:
a first roll driven a first surface speed;
a second roll driven at a second surface speed lower than the first surface speed; and
an elastic belt running over the first roll and the second roll so as to define a first belt section between the first roll and the second roll and a second belt section between the first roll and the second roll opposite the first belt section, the first belt section being thinner than the second belt section.
In addition to use as an ink metering device, the present invention could also be used for applying dampening solution or other liquids distributed to a cylinder in a printing press.